Capitalizing on his high public profile, Koh, 56, who is also a lawyer having appeared on TV many times, topped all opinion polls ahead of the three other competitors vying to become superintendent of the Seoul Metropolitan Office of Education.
That was only until his daughter, who lives in the United States, posted comments on Facebook critical of her father.
In the last poll conducted by broadcasters on May 28, Koh led conservative candidate Moon Young-lin 26.1 percent to 23.5 percent, with liberal-minded contender Cho Hee-yeon lagging far behind with 14.9 percent.
In a country obsessed with education, Koh was widely respected for having passed state-run exams in law, diplomacy and public administration in his 20s.
However, his advantage began evaporating fast on May 31 when Candy Koh, 27, claimed that her father abandoned her and her brother.
On election day, Koh finished in third garnering only 24.25 percent of the vote. Cho, a professor at SungKongHoe University, received 39.08 percent becoming Seoul's top education chief, with incumbent Moon coming in second with 30.65 percent.
Koh's spectacular fall from grace has been a vivid reminder that it is almost impossible for anyone to win public posts in Korea if he or she has a dysfunctional family.
Koh had two children while married to Park Yu-a, a daughter of Park Tae-joon, the founder of the country's largest steelmaker POSCO. But the two divorced in 2002 and Park took the children to the United States. Two years later, Koh remarried.
On May 31, Candy Koh posted messages on her social media page that she and her brother never received educational support from their father.
''I have to inform the citizens of Seoul that he does not qualify for this position,'' she said. ''I am an American and perhaps felt I had no business engaging in dialogue particular to the Korean political scene. However, Koh Seung-duck crossed the line by running for Seoul's superintendent of education seat. I thought it would be to deceive Seoul citizens if I remained silent.''
In response to his daughter's critical comments, Koh held a press conference to tell his side of the story.
He said he agreed with his ex-wife to divorce after they failed to narrow their differences over how their children should be raised.
"My ex-wife took our two children in 1998 after we had fights and said she would take care of them. This hurt me a lot at that time. I was saddened because I lost my children to my ex-wife who is the daughter of a powerful and wealthy family,'' Koh said, denying allegations his daughter made that he abandoned his children.
On the last campaigning day of June 3, Koh made a plea for understanding by apologizing to his daughter. The video clip of him saying ''I am sorry, my daughter'' went viral in cyberspace. However, his last-ditch effort failed.